The main members in the body of a vehicle are an underbody, left and right side-members and a roof. These main members are relative-positioned using a variety of positioning jigs, and are tacked in the positioned state by spot-welding using welding robots, after which final welding is performed.
In conventional vehicle body assembly methods, however, coarse positioning jigs are commonly used to position the main members. In some cases, for example, very large folding screen-type jigs are used to position the side-members and roof In body assembly methods that use such large jigs, provided on the left and right sides of the body assembly line are side-member jigs configured so that they can be shifted back and forth; and provided above the assembly line is a roofjig configured so that it can be raised and lowered. Left and right side-members are transported by conveyers to the fronts of their respective jigs and set in the jigs. The jigs are then moved forward to position them at the prescribed location on the underbody, and the side-members are welded to the underbody. With the side-members still supported by positioning jigs, a roof, supported by a separate jig, is placed across the tops of the side-members, and left and right weld edges of the roof are welded to upper weld edges of the left and right side-members.
Because such folding screen-type jigs are large and heavy, they drive up the initial cost of plant facilities. Also, when there is a change in the model being produced and retooling of the line is required, the existing jigs have to be removed from the jig drive mechanisms and new ones installed. This is extremely time consuming work that brings the production line to a complete stop.
Some conventional vehicle body assembly methods use revolving jigs to speed up the retooling process. When revolving jigs are used, however, because a triangular column is required to support three vehicle body models and a square column is required for four, the equipment is even larger and heavier than when folding screen-type jigs are used, and initial plant facility costs are driven even higher.
Also, the left and right pair of side member jigs and the roofjig are independent of each other, and for the positioning/welding of the roof to the side-members to be accurate, designated vehicle body positions of these three jigs must be accurate with respect to a stationary system referenced to the ground. The side-member jigs are installed in drive means for moving them back and forth and the roofjig is installed in a drive means for moving it up and down; and the positional accuracy of these drive means themselves affect how accurately the roof and side-members can be welded to each other. Because these drive means are required to move the bulky and heavy jigs back and forth or up and down at high rates of speed, maintaining their strict positioning accuracy demands a high level of skill and constant maintenance.
Also, gradual degradation of positioning accuracy over time can occur as moving parts in the drive means become worn.
Also, the left and right pair of side-member jigs and the roofjig are independent of each other, but because the welding edges of all three jigs make contact with the left and right side-members and roof of the vehicle at the same time, this can easily give rise to interfering forces between the three jigs. There is no problem if the forces are balanced between the left and right sides of the vehicle, but variances in the positioning precisions of the jigs, clamping anomalies, etc. can cause forces to be biased toward the left or right side-member. When such force bias occurs, it causes the side-members to tilt leftward or rightward away from the vertical. This distorts the front and rear window openings, which interferes with proper installation of the windshield and rear window glass.
Because inaccurate positioning between the roof and side-members shows up most prominently in distorted front and rear window openings, a very small positioning error can create major problems for installation of the windshield and rear window glass. In roof positioning methods that used large conventional jigs, it was impossible to simultaneously press the roof against the weld edges of the left and right side-members with even pressure because the weld edge of the roof tended to make first contact with only one of the side-member weld edges. When uneven contact between the roof and the left and right side-members occurs, it can cause minor degradation of the accuracy of positioning between the roof and the left and right side-members.
Such problems can be checked for during prototyping, and corrective adjustments made, but it can take a huge amount of time and effort to properly and analyze and isolate the sources of the problems.